Rotating electrical machines can be used in industry for different applications, such as electric motors and generators within the transportation industry, within the process and manufacturing industry, and within the energy industry. Electrical machine is the generic name for a device that converts mechanical energy to electrical energy, converts electrical energy to mechanical energy, or changes alternating current from one voltage level to a different voltage level. Electrical machines as employed in industry fall into three categories according to how they convert energy. Generators convert mechanical energy to electrical energy. Motors convert electrical energy to mechanical energy. Transformers change the voltage of alternating current. Motors and generators commonly belong to the subset of rotating electrical machines.
An electric motor converts electrical energy into mechanical energy. Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force. Electric motors can be found in applications as diverse as industrial applications, small and medium size industrial motors of highly standardized dimensions and characteristics provide convenient mechanical power for industrial uses. Larger electric motors can be used for propulsion of ships in ship propulsion unit applications of the marine industry, for pipeline compressors, and for water pumps with ratings in the millions of watts.
The two main parts of a rotating electrical machine can be described in mechanical terms. A rotor is the rotating part of an electrical machine, and a stator is the stationary part of an electrical machine. The rotor rotates because the wires and magnetic field of the motor can be arranged so that a torque is developed about the rotor's axis. A rotor shaft is a mechanical component for transmitting torque and rotation.
A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on a ferromagnetic rotor which is simply constructed from magnetic material such as soft iron. Torque is generated through the phenomenon of magnetic reluctance.
Synchronous reluctance motors have an equal number of stator and rotor poles. The projections on the rotor can be arranged to introduce internal flux barriers, i.e. holes which direct the magnetic flux along the so-called direct axis. Generally, the axis in the direction of permanent magnet flux is referenced as a direct axis or d-axis, and the axis which is 90 degrees ahead of the direct axis is referenced as a quadrature axis or q-axis. Pole numbers are known to be 4 and 6. When the spaces or notches between the rotor poles can be opposite the stator poles, the magnetic circuit of the motor has a high magnetic reluctance, but when the rotor poles can be aligned with the stator poles the magnetic circuit has a low magnetic reluctance. When a stator pole pair is energized, the nearest rotor pole pair will be pulled into alignment with the energized stator poles to minimize the reluctance path through the machine. As with brushless permanent magnet motors, rotary motion is made possible by energizing the stator poles sequentially causing the rotor to step to the next energized pole.
The synchronous reluctance motor is designed to run on mains frequency alternating current and it uses distributed stator windings similar to those used in squirrel cage induction motors. The rotor, however, needs salient poles to create a variable reluctance in the motor's magnetic circuit which depends on the angular position of the rotor. These salient poles can be created by milling axial slots along the length of a squirrel cage rotor.
The synchronous reluctance motor is not self-starting without the squirrel cage. During run up, it behaves as an induction motor but as it approaches synchronous speed, the reluctance torque takes over and the motor locks into synchronous speed. Once started at synchronous speed, the synchronous reluctance motors motor can operate with sinusoidal voltage. As the rotor is operating at synchronous speed and there are no current-conducting parts in the rotor, rotor losses can be minimal compared to those of an induction motor. Speed control of the synchronous reluctance motors may require an electronic frequency converter.
A stator of a synchronous reluctance motor may have a polyphase stator winding, which has been integrated to grooves in stator core made of magnetically conductive plates. The stator winding produces a rotating field which rotates at the frequency determined by the supply network or by the frequency converter connected to the synchronous reluctance motor. The stator winding of a synchronous reluctance motor is similar to the stator winding of a synchronous motor or to the stator winding of an induction motor. A rotor of a synchronous reluctance motor has been mounted on bearings to rotate within an air gap clearance from the stator.
The functioning of a synchronous reluctance motor is based on an anisotropic rotor structure which rotor has different inductances along its direct and quadrature axes; the inductance along of the direct axis being referenced as Ld and the inductance along of the quadrature axis being referenced as Lq. In its simplest salient pole form, it is similar to the classical synchronous machine without a field winding. However, unlike the synchronous machine, it can only operate at lagging power factor, because all the excitation is from the stator. The linear-start reluctance motors start as induction motors and hence, provided with squirrel cage bars, on the rotor. The stator is similar to the stator of induction counterpart. The motor is accelerated under the influence of induction motor torque and near synchronous speeds, pulled-into synchronism with the synchronously rotating stator field.
The rotor of a synchronous reluctance motor will always try to align its poles with the position that provides minimum reluctance (corresponding to the minimum stored energy in the system). In other words, the torque in a reluctance motor is developed by virtue of a change in the reluctance with the rotor position. The rotor of a synchronous reluctance motor is constructed so that the magnetic permeability is large in the direction of the direct axis and small in the direction of the quadrature axis.
The principle of operation of reluctance machines is based on existence of variable reluctance in the air gap of the machine, high reluctance in the quadrature axis (q-axis) and low reluctance in the direct axis (d-axis). Therefore, for maximizing the power or the torque of a synchronous reluctance motor, the inductance ratio Ld/Lq has to be as great as possible. Therefore, in order to achieve a great inductance ratio Ld/Lq, a number of different structures have been proposed where conducting routes have been designed for magnetic flux along the d-axis and magnetic reluctance barriers have been designed for magnetic flux along the q-axis.
In some structures, the conductive routes for the magnetic flux have been formed with ferromagnetic plates designed so that the ferromagnetic plates have a great magnetic permeability in the direction along the d-axis. Magnetic reluctance barriers have been created by using air or some non-ferromagnetic material.
In JP 2005245052 and U.S. Pat. No. 6,239,526, a rotor of a synchronous reluctance motor have been presented in which the reluctance barriers for the magnetic flux have been formed to the rotor by cutting or carving off parts of the rotor core plates.
In GB 1,109,974, a rotor structure has been presented in which thin electric plates have been constructed on the rotor axle, the plates having a specific preferred magnetic direction having the maximum permeability.
In KR 709301 and U.S. Pat. No. 6,066,904. a rotor of a two-pole synchronous reluctance motor have been presented which rotor has been constructed from directed thin electric plates. In order to achieve the necessary anisotropy of reluctance, slots i.e. magnetic reluctance barriers can be provided in the lamination along the magnetic flux lines in the preferred direction
In JP 11144930, a rotor of a synchronous reluctance motor has been presented in which the rotor core structure is formed by stacking stripe-shaped metal pieces and metallurgically joining a magnetic metal material and a non-magnetic metal material.
In WO 96/42132, a rotor of a synchronous reluctance motor has been presented in which the rotor core structure is constructed from magnetic material and non-magnetic material, and which materials can be covered with a layer of non-magnetic conducting material.
There can be some problems when using rotor core structures constructed from magnetic material and non-magnetic material, which materials can be laminated with a layer of non-magnetic conducting material. These laminated rotor core structures cannot withstand high centrifugal forces.
So far, the high speed motor rotors have utilized induction technologies (coated solid rotors) or synchronous permanent magnet technologies. In order to cope with the ultra-high centrifugal forces, the conductive copper coating should be explosion welded to the solid iron surface. In permanent magnet rotors, the magnets have to be secured using thick carbon fiber bandage, which is adverse in thermal sense. Both of these technologies can be difficult and expensive to manufacture. The permanent magnet rotor also suffers from vulnerability to eddy current losses in magnets.
The problem therefore is to find a configuration and materials which can produce the reluctance effect and withstand the centrifugal forces while still keeping the harmonic losses on the rotor surface to minimum.
There is a demand for a method for manufacturing a rotor of a synchronous reluctance motor which rotor would produce the reluctance effect, withstand the centrifugal forces and keep the harmonic losses on the rotor surface to minimum when compared to the prior art solutions. Likewise, there is a demand for a rotor of a synchronous reluctance motor producing the reluctance effect, withstanding the centrifugal forces and keeping the harmonic losses on the rotor surface to minimum when compared to the prior art solutions; and also a demand for a synchronous reluctance motor with a rotor having such characteristics.